A wireless ad-hoc network, also sometimes referred to as a mobile ad-hoc network (MANET), is known to comprise a set of nodes connected by wireless links. A typical example of an ad-hoc network is a wireless sensor network, where the nodes are sensors that gather environmental data and send the information to computational nodes, otherwise known as sink nodes, for further processing, or to base stations for relay to a wired network. Such networks may be deployed, for example, in hazardous locations such as in disaster areas (e.g., earthquakes, fires, etc.) to aid rescue efforts, in areas for mineral or oil prospecting, and in battlefields for defence applications.
In some applications the sink node has a fixed position. In other applications however, such as perimeter monitoring for physical intrusion detection, or for home automation, the sink should be mobile. For example in the case of perimeter monitoring the sink node may be carried by a security guard moving around the perimeter. In the case of home automation the sink node may be carried in a universal remote control by the user living in the home.
The topology of an ad-hoc network is typically dynamic since nodes are free to move randomly and organize themselves arbitrarily. The nodes in an ad-hoc network typically attempt to communicate information to the sink node by relaying packets. How to route a packet to the sink poses a problem. A broadcast packet would find the sink but at the expense of flooding the network. Conversely a probabilistic forwarding to an estimation of the sink's random position would only find the sink within some probability bounds. It is therefore necessary to design efficient routing protocols to meet a variety of performance objectives given such a communication environment.
Since in a wireless sensor network, sensor nodes operate with an isolated power supply, such as a battery, one such performance objective is energy efficiency. This is crucial to the operation of the network since the lifetime of a sensor node in the network is dependent on the lifetime of the battery.
Most existing routing protocols for ad hoc networks do not take into account the constraints of a wireless sensor network, in particular the characteristic that there is only one or very few sink nodes in the network to which communication is directed.
Routing protocols such as AODV (Ad Hoc On Demand Distance Vector) and DSR (Dynamic Source Routing) take mobility of the network nodes into account but are not energy efficient. Protocols which are designed to be energy efficient, such as gradient based routing, are not designed to support the mobility of the sink node.
US 2005/0135360 describes a method of providing a routing protocol in a sensor network. Energy information is contained in a request message of an AODV protocol, thereby allowing the neighbouring nodes and the nodes on the path to know their energy states.
US 2005/0157698 describes a method of updating routing information in a wireless sensor network. A sensor node collects information of a target and broadcasts a routing request message when the information of the target is not received anymore. Route information may be updated based on the information contained in the routing request massage from the sensor node.
The LER (Last Encounter Routing) protocol is used in ad hoc networks and uses information based on the time and location of a node with a destination node. Geographic routing is used to route a packet to the location of the last encounter with the destination node.
It is therefore an aim of the present invention is to maximize the energy efficiency in a sensor network.